Vibrating beam force sensors are quite well known--the basic idea was described over twenty years ago--and since the early 1960's, these devices (in which in essence a beam or strip of a piezoelectric material mounted at either end is piezoelectrically driven into flexural vibration while under tension, a change in the vibrational frequency indicating a change in the tensioning forces), have found a wide range of uses.
Unfortunately the simplest form of the device, a single strip-like beam mounted at either end--tends to have a relatively low Q (the factor used to indicate the amount of energy locked in the vibrating structure relative to the amount of energy that must be fed in to maintain the vibrations), and the energy is lost mainly by transfer to the mountings at either end. Much effort has gone into designing beam-like structures that do not suffer from the low Q problem--that do not cause a large proportion of the input energy to be passed to and absorbed by the mountings--and much of this effort has centered on the idea of providing some sort of counterbalanced vibrating element such that the vibrations of both this and the beam effectively cancel each other at the mountings, so that no energy is transferred to the mountings and the whole structure has a high Q.
Although these counterbalanced, or compensated, structures do undoubtedly have the desired high Q, they are nevertheless all quite complex, and difficult and costly to manufacture. One such structure, put forward in the early 1960's, uses the tuning fork principle (two similar members vibrating to and from each other, in antiphase) by having two beams mounted at their ends in common mountings and disposed one above but spaced from the other. Like the arms of a tuning fork, the two beams flexurally vibrate in their common plane--that is, towards and away from each other. Because they are in antiphase (180.degree. out of phase) the vibrations sent to each mounting by one beam are exactly equal but opposite to those sent by the other beam, and so they cancel out, and no energy is transferred to the mountings. Another structure, suggested in the early 1970's, tries to solve the Q problem by securing the beam to each mounting via a torsion member at right angles to the beam's long axis, and by then providing a counterweight beam section beyond each torsion member. Yet another structure, also suggested in the 1970's, proposed a variant on the last one, mounting the beam at each end via two "isolator springs" spaced above and below the beam plane and then having two counterweights extending from these towards the beam center. Structures such as these are not only difficult and expensive to manufacture from the raw piezoelectric material blank, but in some cases the positioning thereon of the necessary electrodes (both by which the beams can be driven and by which the vibration's actual frequency can be observed) is made particularly irksome because of the complex shapes involved.
It appears that all of the high Q structures suggested so far involve balancing beams or counterweights that are in the vibrational plane of the "main" beam, and flex in that plane. This seems to have made all these structures unnecessarily complex, and it is the hope of the present invention that it can provide a mechanically simpler, and cheaper, but no less efficient beam structure by placing counterbalancing beams not above and below the main beam but on either side thereof.